Electromagnetic borehole telemetry system incorporating a conductive borehole tubular

ABSTRACT

An electromagnetic borehole telemetry system for transmitting information between a borehole transceiver and a surface transceiver located at or near the surface of the earth. Tubulars, such as steel casing and liners, are typically set within the well borehole to stabilize the wall of the borehole and to assist in hydraulically isolating penetrated formations. The invention utilizes these tubulars cooperating with one or more signal wires to reduce attenuation and noise in signals transmitted between the borehole and surface transceivers. The one or more signal wires are typically disposed within an annulus formed by the borehole wall and the outside surface of the casing. The one or more signal wires are connected at one end to one or more connection terminals positioned preferably near the bottom a tubular string. Opposing ends of the one or more signal wires are connected to one or more terminals of the surface transceiver. By minimizing signal attenuation and noise, the telemetry system can be effectively used at greater depths in the borehole. The telemetry system can be embodied in a measurement-while-drilling system, a formation testing system, a production monitoring system and other system requiring communication between a downhole assembly and the surface of the earth.

This invention is directed toward electromagnetic borehole telemetrysystem for transmitting information between a borehole transceiver and atransceiver at or near the surface of the earth. More specifically, theinvention is directed toward an electromagnetic telemetry system whichuses a signal wire cooperating with conductive tubular within theborehole to reduce signal attenuation and enhance signal to noise ratiothereby increasing the depth within the borehole at which the telemetrysystem can operate efficiently. The borehole transceiver cooperates withone or more sensors, and is typically disposed in a downhole assemblyused to drill a borehole, to measure drilling and formation parameters,to test potential of a well borehole penetrating a hydrocarbon bearingformation, or to monitor production of a hydrocarbon or other fluidproducing well.

BACKGROUND OF THE INVENTION

The creation of a hydrocarbon producing well can be broadly classifiedin three stages. The first stage includes the drilling of the wellborehole, where it is desirable to measure properties of earthformations penetrated by the borehole and to steer the direction of theborehole while drilling. The second stage includes testing of formationspenetrated by the borehole to determine hydrocarbon content adproducability. The third stage includes monitoring and controllingproduction typically throughout the life of the well. Operations in allstages typically employ a downhole assembly that contains one or moresensors responsive to stage related drilling, formation, or productionparameters of interest Response data from the one or more sensors aretelemetered to the surface of the earth and received by a secondtransceiver for processing and interpretation. Conversely, it isdesirable to transmit data via the surface transceiver to the boreholetransceiver to control stake related drilling, testing or productionoperations.

In many of the stage operations discussed above, it is not operationallyfeasible to use a “hard wire” communication link, such as one or moreelectrical or fiber optic conductors, between the borehole transceiverand the surface transceiver. When wire communication links are notfeasible, electromagnetic (EM) telemetry systems offer one means forcommunicating between borehole and surface transceivers. Datatransmission rates using EM communication links are typically much lowerthan those of hard wire communication links. Signal attenuation in EMcommunication links is typically much higher than that in hard wirecommunication links, for a given operational depth within a borehole.

As mentioned above, direct or hard wire communication links for datatelemetry are often operationally impractical in many well stageoperations. This is especially true in the borehole drilling stage,where measures of parameters of formations penetrated by the boreholeare of interest. Systems for measuring such geophysical and otherparameters within the vicinity of a well borehole typically fall withintwo categories. The first category includes systems that measureparameters after the borehole has been drilled. These systems includewireline logging, tubing conveyed logging, slick line logging,production logging, permanent downhole sensing devices and othertechniques known in the art. Memory type or hard wire communicationlinks are typically used in these systems. The second category includessystems that measure formation and borehole parameters while theborehole is being drilled. These systems include measurements ofdrilling and borehole specific parameters commonly known as“measurement-while-drilling” (MWD), measurements of parameters of earthformation penetrated by the borehole commonly known as“logging-while-drilling” (LWD), and measurements of seismic relatedproperties known as “seismic-while-drilling” or (SWD). For brevity,systems that measure parameters of interest while the borehole is beingdrilled will be referred to collectively in this disclosure as “MWD”systems. Within the scope of this disclosure, it should be understoodthat MWD systems also include logging-while-drilling andseismic-while-drilling systems.

A MWD system typically comprises a downhole assembly operationallyattached to a downhole end of a drill string. The downhole assemblytypically includes at least one sensor for measuring at least oneparameter of interest, control and power elements for operating thesensor, and a borehole transceiver for transmitting sensor response tothe surface of the earth for processing and analysis. The downholeassembly is terminated at the lower end with a drill bit. A rotarydrilling rig is operationally attached to an upper end of the drillstring. The action of the drilling rig rotates the drill string anddownhole assembly thereby advancing the borehole by the action of therotating drill bit. A surface transceiver is positioned remote from thedownhole assembly and typically in the immediate vicinity of thedrilling rig. The surface transceiver receives telemetered data from thedownhole transceiver. Received data are typically processed usingsurface equipment, and one or more parameters of interest are recordedas a function of depth within the well borehole thereby providing a“log” of the one or more parameters. Hard wire communication linksbetween the borehole and surface transceivers are operationallydifficult because the downhole assembly containing the boreholetransceiver is rotated typically by the drill string.

In the absence of a hard wire link, several techniques can be used as acommunication link for the telemetry system. These systems includedrilling fluid pressure modulation or “mud pulse” systems, acousticsystems, and electromagnetic systems.

Using a mud pulse system, a downhole transmitter induces pressure pulsesor other pressure modulations within the drilling fluid used in drillingthe borehole. The modulations are indicative of data of interest, suchas response of a sensor within the downhole assembly. These modulationsare subsequently measured typically at the surface of the earth using areceiver means, and data of interest is extracted from the modulationmeasurements. Data transmission rates are low using mud pulse systems.Furthermore, the signal to noise ratio is typically small and signalattenuation is large, especially for relatively deep boreholes.

A downhole transmitter of an acoustic telemetry induces amplitude andfrequency modulated acoustic signals within the drill sting. The signalsare indicative of data of interest. These modulated signals are measuredtypically at the surface of the earth by an acoustic receiver means, anddata of interest are extracted from the measurements. Once again, datetransmission rate, the signal to noise ratio of the telemetry system issmall, and signal attenuation as a function of depth within the boreholeis large.

Electromagnetic telemetry systems can employ a variety of techniques.Using one technique, electromagnetic signals are modulated to reflectdata of interest. These signals are transmitted from a downhole EMtransceiver, through intervening earth formation, and detected using asurface transceiver that is typically located at or near the surface ofthe earth. Data of interest are extracted from the detected signal.Using another electromagnetic technique, a downhole transceiver createsa current within the drill sting and the current travels along the drillstring. This current is typically created by imposing a voltage across anon-conducting section in the downhole assembly. The current ismodulated to reflect data of interest. A voltage between the drillingrig and a remote ground is generated by the current and is measured by atransceiver, which is at the surface of the earth. The voltage isusually between a wire attached to the drilling rig or casing at thesurface and a wire which leads to a grounded connection remote from therig. Again, data of interest are extracted from the measured voltage.When data are sent from the surface transceiver to the downholetransceiver, voltage is applied between a point on the rig and a remoteground. This, in turn, creates a current that travels along the drillstring and casing, and is detected by the downhole transceiver in theform of a voltage across the non-conducting section of the downholeassembly.

SUMMARY OF THE INVENTION

This present invention is directed toward an electromagnetic (EM ) wellborehole telemetry system for transmitting information between a“borehole” EM transceiver, disposed preferably within a downholeassembly in the borehole, and a “surface” EM transceiver positioned ator near the surface of the earth. One or more conductive tubulars, suchas steel casing and liners, are typically set within the well tostabilize the wall of the borehole and to assist in hydraulicallyisolating penetrated formations, as is known in the art. The inventionutilizes these conductive tubulars within the borehole. Using a stringof casing as an example, one or more insulated conductor wires,hereafter referred to as “signal” wires, are preferably disposed withinan annulus formed by the borehole wall and the outside surface of thecasing. In a preferred embodiment, the one or more signal wires areelectrically connected at one end to one or more casing connectionterminals, providing an electrode means, positioned preferably near thebottom of the casing string. Opposing ends of the one or more signalwires are connected to one or more signal terminals of the surface EMtransceiver. A remote ground wire may or may not be used. In anotherembodiment, the end(s) of the signal wire(s) is(are) connected to anelectrode means that is not electrically connected to the casing but maybe conveyed by the casing via a mechanical connection to a point(s)downhole in the annulus between the casing and the borehole wall. Thiselectrode means could be a section of bare wire or a conducting platewhich, by contact with the material in the annulus between the casingand the borehole wall, will be at the potential of that same annulusregion. In yet another embodiment, the signal wire(s) and electrodemeans are conveyed by means other that the casing, such as a weightedend to a point(s) downhole in the annulus between the casing ad theborehole wall.

For purposes of initial discussion, assume that only one signal wire iselectrically connected between a single casing connector terminal nearthe bottom of the casing and a single surface EM transceiver terminal.EM transceiver ground is connected to a remote ground by a by a groundwire. In the prior art, the EM signal is attenuated by interveningformation and borehole material between the surface and borehole EMtransceivers. By using the signal wire, the transmitted EM signal issignificantly attenuated only by intervening formation and boreholematerial between the borehole EM transceiver and the casing connectionterminal located downhole. Because preferably a high impedance voltagemeasurement is now made at a point downhole on or beside the casing atthe electrode means signal attenuation between the casing connectionterminal and the surface EM transceiver is essentially eliminated. Thehigh impedance voltage measurement which is preferably made causes verylow or negligible current to flow in the signal wire, therefore, thereis negligible attenuation within the signal wire. Stated another way,the effective distance between the surface and borehole EM transceiversis reduced. By utilizing the signal wire, overall signal attenuation isreduced significantly compared to attenuation of an EM signaltransmitted directly between the borehole EM transceiver and the surfaceEM transceiver. Electromagnetic noise induced at or near the surface isalso minimized since the signal wire is not attached at the surface, butis electrically connected to the casing downhole. In summary, the EMtelemetry system is configured to minimize signal attenuation and toenhance signal-to-noise ratio. These features increase the depth withinthe borehole at which the telemetry system can operate efficiently.

Embodiments of the telemetry system can be varied as will be discussedin detail in subsequent sections of this disclosure. Details ofoperating principles of the surface and borehole transceivers aredisclosed in U.S. Pat. No. 4,684,946 (transmitter) and U.S. Pat. No.5,394,141 (long dipole antenna), and are hereby entered into thisdisclosure by reference.

The borehole EM transceiver cooperates with one or more sensorstypically disposed in a downhole assembly. The downhole assembly cancomprise a MWD element used in the first operational stage of drillingthe well borehole. In an alternate embodiment, the downhole assembly cancomprise a testing element used in the second operational stage to testpotential of a hydrocarbon bearing formation penetrated by the borehole.In yet another alternate embodiment, the downhole assembly can comprisea monitor element used in the third operational stage to monitorproduction of a hydrocarbon or other fluid producing well. For purposesof disclosure, the EM telemetry system embodied as a MWD telemetrysystem will be described in detail. It should be understood, however,that the system can be embodied with equal effectiveness in a secondstage formation testing system or a third stage well monitoring andproduction system.

Embodied in a MWD system, the borehole EM transceiver is typicallydisposed within a downhole assembly that is operationally attached to adownhole end of a drill string. In addition, the downhole assemblytypically includes at least one sensor for measuring at least oneborehole or formation parameter of interest, control and power elementsfor operating the sensor and the borehole EM transceiver. The downholeassembly is term ted at the lower end with a drill bit A rotary drillingrig is typically attached to an upper end of the drill string The actionof the drilling rig rotates the drill string and downhole assemblythereby advancing the borehole by the action of the attached drill bit.One or more intermediate strings of casing are typically “set” withinthe borehole as it is advanced by the drill bit. The signal wire iselectrically connected at one end to a casing connection terminal,preferably near the bottom of the casing, and at a second end to theface EM transceiver at or relatively near the surface of the earth. Asecond terminal of the surface EM transceiver is grounded at a pointremote from the drilling rig, or alternately electrically connected toanother part of the casing. The voltage is measured between these twotransceiver inputs. The surface EM transceiver receives telemetered datavia the measured voltage, indicative of sensor response, from theborehole EM transceiver. Received data are typically processed using asurface processor and converted to well borehole or formation parametersof interest. Data can also be transmitted from the surface to thedownhole assembly via the surface EM transceiver, Parameters of interestare recorded at the surface as a function of depth within the wellborehole thereby providing a “log” of the parameters of interest. Asdiscussed previously, a hard wire communication link directly connectingthe borehole and surface EM transceivers is operationally difficultbecause the downhole assembly containing the borehole transceiver isrotated typically by the drill string.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects the present invention are obtained and can be understood indetail, more particular description of the invention, briefly summarizedabove, may be had by reference to the embodiments thereof which areillustrated in the appended drawings.

FIG. 1 is a conceptual illustration of the basic elements of theinvention;

FIG. 2 shows the EM telemetry system embodied in a MWD system;

FIG. 3 shows the EM telemetry system again embodied in a MWD system, butwith the lower end of the signal wire connected to an electrode which iselectrically insulated from the casing;

FIG. 4 shows the FM telemetry system once again embodied in a MWD systemthat employs two signal wires;

FIG. 5 shows the EM telemetry system employing two signal wires whereinthe surface EM transceiver is disposed within the annulus defined by theouter surface of casing and the wall of the borehole;

FIG. 6 shows an offshore embodiment of the EM telemetry system whereinthe surface EM transceiver is located beneath a body of water; and

FIG. 7 shows another offshore embodiment of the EM telemetry system thatis again similar to the land embodiment of the system shown in FIG. 2,wherein the surface EM transceiver is located above surface of the bodyof water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This present invention is directed toward an electromagnetic (EM)borehole telemetry system for transmitting information between a“borehole” EM transceiver, disposed preferably within a downholeassembly in the borehole, and a “surface” EM transceiver at or near thesurface of the earth. It is noted that the “surface” EM transceiver neednot be located on the surface of the earth, but it is always disposedabove or “up-hole” with respect to the borehole EM transceiver.

FIG. 1 is a conceptual illustration of the basic elements of theinvention, which is identified as a whole by the numeral 10. The system10 operates at a low frequency, typically in the frequency range lessthan 100 Hertz (Hz). A string of conductive tubular, such as steelcasing, is shown disposed within a borehole 19 penetrating earthformation 13. Although only a single string of tubular 18 is shown, itshould be understood that the methods and apparatus of the invention armequally applicable to boreholes containing two or more concentric stringof tubulars such as casings, liners, screens and the like. A downholeassembly 20 is sown disposed within the borehole 19 below the tubularstung 18. The downhole assembly comprises a borehole EM transceiver 22,which is typically connected operationally to at least one sensor 24.The downhole assembly 20 can comprise a MWD element, wherein the one ormore sensors 24 respond to formation and borehole parameters. In analternate embodiment, the downhole assembly 20 can comprise a testingelement, wherein the one or more sensors 24 respond to the potential ofa hydrocarbon bearing formation penetrated by the borehole 19. In yetanother alternate embodiment, the downhole assembly 20 can comprise oneor more sensors 24 used to monitor production of a hydrocarbon or otherfluid produced from the formation 13. It should be understood that thedownhole assembly 20 can be embodied to measure or monitor additionalparameters associated with the drilling, completion and production ofthe well borehole 19.

Still referring to FIG. 1, a signal wire 28 is shown disposed within anannulus defined by the outer surface of the tubular 18 and the wall 16of the borehole 19. The signal wire is electrically connected at one endto a casing connection terminal 15 positioned preferably near the bottomof the tubular string 18. The opposing end of the signal wire 28 iselectrically connected to a terminal 27 of a surface EM transceiver 26disposed at or near the surface 14. If two or more strings of tubularsare used, the signal wire 28 can be disposed within an annulus definedby two strings of tubulars Alternately, the signal wire can be disposedinside the inner most sting of tubular.

It is noted that the connection of the signal wire 28 at casingconnection terminal 15 can be a physical electrical or mechanicalconnection. Examples of physical connections include, but are notlimited to, a bolt that connects the signal wire 28 directly to thecasing, a flange welded to the casing and to which the signal wire isbolted, a flange welded to the casing and to which the signal wire iswelded, a weld connecting the signal wire directly to the casing.Alternately the connection can be an electrode means in contact with thematerial between the casing and the borehole wall which is not connectedto the casing.

Again referring to FIG. 1, electromagnetic signals, typically indicativeof the response of the one or more sensors 24, are transmitted from theborehole EM transceiver 22 to the surface EM transceiver 26. Conversely,control or other signals are transmitted from the surface EM transceiver26 to the borehole EM transceiver 22. The casing 18 alters the path ofan EM signal transmitted between the surface EM transceiver 26 andborehole EM transceiver 22. By using the signal wire 28, the transmittedEM signal is significantly attenuated only by intervening formation andborehole material between the borehole EM transceiver 22 and the casingconnection terminal 15. Signal attenuation between the casing connectionterminal 15 and the surface EM transceiver 26 is essentially eliminatedsince signal attenuation within the signal wire is negligible becausecurrent within the wire is minimal. The “effective” distance between thesurface EM transceiver 26 and borehole EM transceiver 22 is reduced by adistance indicated by the numeral 131.

Once again referring to FIG. 1, the surface EM transceiver 26 isgrounded by a ground wire 30 at a ground point 32 which is remote aspractical from the well borehole 19. The surface EM transceiver 26 isresponsive to voltage between the casing connection 15 and the groundpoint 32. Signals from the one or more sensors 24 are received by thesurface EM transceiver 26 and are transmitted by a link 44 to aprocessor 34. The processor converts these signals into parameters ofinterest. The processor 34 also provides power for the surface EMtransceiver 26 and means to input control signs to be telemetered viathe surface EM transceiver to the borehole EM transceiver 22. Controlsignals are sensed as voltages measured using the borehole EMtransceiver 22.

By utilizing the signal wire 28 as illustrated in FIG. 1, overall signalattenuation is reduced significantly compared to attenuation of an EMsignal transmitted directly between the borehole EM transceiver 22 andthe surface EM transceiver 26. In summary, the EM telemetry system isconfigured to minimize signal attenuation and to enhance signal-to-noiseratio thereby increase the depth within the borehole 19 at which thetelemetry system 10 can operate efficiently.

FIG. 2 illustrates the EM telemetry system 10 embodied in a MWD systemThe borehole EM transceiver 22 is disposed within a downhole assembly 20that is operationally attached to a downhole end of a drill string 40.In addition, the downhole assembly 20 typically includes at least onesensor 24 for measuring at least one parameter of the formation 13 or adrilling parameter, control and power elements (not shown) for operatingthe sensor 24 and the borehole EM transceiver 22. The downhole assembly20 is terminated at the lower end with a drill bit 31. A rotary drillingrig 38, which is well known in the art, is typically attached to anupper end of the drill string. The action of the drilling rig 38typically rotates the drill string 40 and downhole assembly 20 withattached drill bit 31 thereby advancing the borehole 19. Intermediatestrings of casing are typically “set” within the borehole 19 as it isadvanced by the drill bit 31. One such string of casing 18 isillustrated, with the drill string 40 traversing the inside of thecasing. A signal wire 28 is attached at one end to a casing connectionterminal 15, preferably near the bottom of the casing 18, and at asecond end to a terminal 27 of the surface EM transceiver 26 which ispositioned at or relatively near the surface 14 of the earth. Thesurface EM transceiver 26 receives telemetered data, indicative ofresponse of the one or more sensors 24, from the borehole EM transceiver22. The surface EM transceiver 26 is again grounded at a remote point 32by a ground wire 30. Received data are transferred by link 44 to asurface processor 34, where these data are converted to well borehole orformation parameters of interest. Data can also be to transmitted fromthe surface to the downhole assembly 20 via the surface EM transceiver26. Parameters of interest are recorded at the surface as a Action ofdepth within the well borehole thereby providing a “log” of the one ormore parameters of interest.

FIG. 3 shows the EM telemetry system 10 again embodied in a MWD s Theembodiment is similar to the embodiment show in FIG. 2, except that thelower end of the of the signal wire 28 is attached at casing connectionterminal 15 to an electrode structure 18 b which is insulated from thecasing 18 by a section or “joint” of non conducting casing 18 a. Usingthis embodiment, the electrode structure 18 b is closer to the potentialof the casing or the drill string 40 immediately inside the casingthereby reducing filer the attenuation of EM signals between theborehole EM transceiver 22 and the surface EM transceiver 26. Details ofthe use of a non conducting joint of casing in an EM telemetry systemare disclosed in U.S. Pat. No. 5,163,714, which is hereby entered intothis disclosure by reference. Other elements shown in FIG. 3 arefunctional the same as corresponding elements shown and discussed inFIG. 2.

FIG. 4 shows the EM telemetry system 10 once again embodied in a system.The embodiment is similar to those shown and discussed in FIGS. 2 and 3,except that two signal wires are employed A first signal wire 28 a isattached at one end to a casing connection terminal 15 a, againpreferably near the bottom of the casing 18, and at a second end to aterminal 27 a of the surface EM transceiver 26 at the surface 14 of theearth. A second signal wire 28 b is attached at one end to a casingconnection terminal 15 b, which is axially spaced above the casingconnection terminal 15 a on the casing 18, and at a second end toterminal 27 b of the surface EM transceiver 26. Using this arrangement,signals input into the surface EM transceiver 26 are dependent, onlyupon EM signals generated in the casing by the borehole EM transceiver22. The ground wire 30 shown in embodiments of FIGS. 2 and 3 is notrequired. Any surface noise between a remote ground (see 32 in FIGS. 2and 3) and the surface EM transceiver 27 is, therefore, eliminated. Thenon-conducting joint 18 a, illustrated with broken lines, is optional inthis embodiment of the system. Other elements shown in FIG. 4 arefunctional the same as corresponding elements shown and discussed inFIGS. 2 and 3. The two signal wires going from the connection 28 b andthe terminal 27 of the surface EM transceiver 26 are preferably atwisted pair or a coaxial cable.

All signal wires 28, 28 a and 28 b am preferably rugged to withstandrough operations conditions and harsh borehole conditions. Armoredwireline cable meets such requirements.

FIG. 5 shows yet another embodiment of the EM telemetry system 10. Thisembodiment can be used in conjunction with a MWD system, but elements ofthe drilling rig have been omitted for purposes of clarity. Thisembodiment, as well as previously discussed embodiments, can also beused in conjunction with formation testing systems and productionmonitoring systems. The two signal wire embodiment is similar to thatshown in FIG. 4, except that the surface EM transceiver 26 has also beendisposed within the annulus defined by the outer He of the casing 18 andthe wall 16 of the borehole 19. In this embodiment, power and controlsignals are supplied from the processor 34 to the surface EM transceiver26 via the link 44. Signals received by the surface EM transceiver 26are transmitted to the processor 34 via the link 44. Data transmitted tothe borehole EM transceiver 22 are first transmitted from the processor34 to the surface EM transceiver 26 via the link 44. This embodimentfurther reduces surface noise by processing the telemetry signals in anelectrically “quiet” environment of the borehole 19 rather than at thesurface 14.

FIG. 6 shows an offshore embodiment of the EM telemetry system 10 thatis similar to the land embodiment of the system shown in FIG. 2. Again,this embodiment can be used in conjunction with a MWD system, butelements of the drilling rig have been omitted for purposes of clarity.This embodiment can also be used in conjunction with formation testingsystems and production monitoring systems discussed previously. Thesurface EM transceiver 26 is located on or near a surface 14 a, whichlies beneath a body of water 42. A tubular string, such as casing 18,extends from the surface 14 b of the water body 42 into a borehole 19penetrating earth formation 13 beneath the water. A signal wire 28disposed in an annulus defined by the surface of the casing 18 and theborehole wall 16. One end of the signal wire 28 is again attached to acasing connection terminal 15, preferably near the bottom of the casing18, and at a second end to a terminal 27 of the surface EM transceiver26. The surface EM transceiver 26 is disposed at or relatively near theearth spice 14 a beneath the body of water 42. Once again, the surfaceEM transceiver 26 receives telemetered data, indicative of response ofthe one or more sensors (not shown), from the borehole EM transceiver22. The surface EM transceiver 26 is grounded at a remote, underwaterpoint 32 by a ground wire 30. Data received by the surface EMtransceiver 26 are transferred by link 44 to a surface processor 34disposed above the water surface 14 b, where these data are converted towell borehole or formation parameters of interest. Once again, data canbe transmitted from the processor 34 to the surface EM transceiver 26via the link 44, and subsequently to the borehole EM transceiver 22 viapreviously discussed EM signal transmission. The link 44 also serves asa means for powering and controlling the surface EM transceiver 26.

FIG. 7 shows another offshore embodiment of the EM telemetry system 10that is again similar to the land embodiment of the system shown in FIG.2. As mentioned previously, this embodiment can be used in conjunctionwith a MWD system or alternately in conjunction with formation testingsystems and production monitoring systems discussed previously. Thesurface EM transceiver 26 is located above surface 14 b of the body ofwater 42 The casing 18 again extends from the surface 14 b of the waterbody 42 into the borehole 19 penetrating earth formation 13 beneath thewater. A signal we 28 traverses the water 42 between the surfaces 14 band 14 a, and is then disposed in the annulus defined by the surface ofthe casing 18 and the borehole wall 16. The signal wire 28 is againattached at one end to a casing connection terminal 15, preferably nearthe bottom of the casing 18, and at a second end to a terminal 27 of thesurface EM transceiver 26. As in previous embodiments, the surface EMtransceiver 26 receives telemetered data, indicative of response of theone or more sensors (not shown), from the borehole EM transceiver 22.The surface EM transceiver 26 is grounded at a remote, underwater point32 by a ground wire 30 that traverses the water body 42. Data transferbetween the surface EM transceiver 26 and the borehole EM transceiver 22has been discussed previously.

Comparing the onshore embodiments of the EM telemetry system 10 shown inFIGS. 6 and 7, positioning the surface EM transceiver 26 beneath thesurface 14 b of the water reduces noise but introduces some operationaldifficulties in powering and maintaining the surface EM transceiverunder water. Conversely, positioning the surface EM transceiver 26 abovethe water surface 14 b is operationally advantageous, but is moresusceptible to nose than the embodiment shown in FIG. 6.

It should be understood that embodiments of the EM telemetry system 10shown in FIGS. 4 and 5 can also be adapted for offshore operations bycombining these embodiments with features shown in the embodiments ofFIGS. 6 and 7.

While the foregoing disclosure is directed toward the preferredembodiments of the invention, the scope of the invention is defined bythe claims, which follow.

1. A telemetry system for transmitting an electromagnetic signal withina borehole, the system comprising: (a) a borehole EM transceiver; (b) asurface EM transceiver; and (c) a signal wire with a first endcomprising electrode means connected within said borehole to the outersurface of a tubular, and a second end electrically connected to saidsurface EM transceiver; wherein (d) said signal wire reduces attenuationof said signal transmitted between said surface EM transceiver and saidborehole EM transceiver.
 2. The telemetry system of claim 1 furthercomprising a ground wire with a first end electrically connected to saidsurface EM transceiver and a second end connected to a ground pointremote foam said surface EM transceiver.
 3. The telemetry system ofclaim 2 wherein said signal wire is disposed in the annulus between saidtubular and the wall of said borehole.
 4. The telemetry system of claim3 wherein said first end of said signal wire is connected near the lowerend of said tubular.
 5. The telemetry system of claim 3 where said firstend of said signal wire is electrically connected to the outer surfaceof said tubular.
 6. The telemetry system of claim 3 where said first endof said signal wire is mechanically connected to the outer surface ofsaid tubular.
 7. The telemetry system of claim 3 wherein said surface EMtransceiver is positioned above a surface of earth through which saidborehole penetrates.
 8. The telemetry system of claim 3 wherein: (a)earth surface through which said borehole penetrates is covered by abody of water; (b) said tubular extends above a surface of said body ofwater; and (c) said borehole EM transceiver is disposed above said earthsurface and within said body of water.
 9. The telemetry system of claim3 wherein: (a) earth surface through which said borehole penetrates iscovered by a body of water; (b) said tubular extends above a surface ofsaid body of water; and (c) said borehole EM Receiver is disposed abovesaid surface of water.
 10. The telemetry system of claim 3 wherein saidfirst end of said signal wire is affixed to an electrode structure thatis electrically insulated from said tubular by a non conducting sectionof tubular.
 11. The telemetry system of claim 1 comprising a secondsignal wire, with a first end comprising electrode means disposed withinsaid borehole axially spaced from said first end of said signal wire anda second end electrically connected to said surface EM transceiver,wherein said signal wire and said second signal wire cooperate to reducesurface noise in said signal.
 12. The telemetry system of claim 11wherein said signal wire and said second signal wire are disposed withinan annulus defined by an outer surface of said tubular and the wall ofsaid borehole.
 13. The telemetry system of claim 12 wherein said surfaceEM transceiver is disposed within said annulus.
 14. The telemetry systemof claim 12 wherein said surface EM transceiver is positioned aboveearth surface through which said borehole penetrates.
 15. Ameasurement-while-drilling system for measuring a parameter of interest,said system comprising: (a) a downhole assembly comprising a sensor,wherein said downhole assembly is terminated at a lower end by a drillbit and at an upper end by a drill string operationally attached to adrilling rig; (b) an electromagnetic telemetry system for transmittingan electromagnetic signal indicative of a response of said sensor, saidtelemetry system comprising (i) a surface EM transceiver for receivingsaid signal, (ii) a borehole EM transceiver for transmitting said signalwherein said borehole EM transceiver is disposed within said downholeassembly and operationally connected to said sensor, and (iii) a signalwire with a first end comprising electrode means connected within saidborehole to the outer surface of a casing string, and a second endelectrically connected to said surface EM transceiver; and (c) aprocessor cooperating with said surface EM transceiver by means of alink to convert said signal into said parameter of interest; wherein (d)said signal wire reduces attenuation of said signal transmitted betweensaid surface EM transceiver and said borehole EM transceiver.
 16. Themeasurement-while-drilling system of claim 15 further comprising aground wire with a first end electrically connected to said surface EMtransceiver and a second end connected to a ground point remote fromsaid drilling rig.
 17. The measurement-while-drilling system of claim 16wherein said signal wire is disposed within an annulus defined by anouter surface of said casing string and the wall of said borehole. 18.The measurement-while-drilling system of claim 11 wherein said surfaceEM transceiver is positioned above earth surface through which saidborehole penetrates.
 19. The measurement-while-drilling system of clam17 wherein: (a) earth surface through which said borehole penetrates iscovered by a body of water; (b) said casing sting extends above asurface of said body of water; (c) said borehole EM transceiver isdisposed above said earth surface and within said body of water; and (d)said processor is disposed above said surface of said water.
 20. Themeasurement-while-drilling system of claim 17 wherein: (a) earth surfacethrough which said borehole penetrates is covered by a body of water;(b) said casing extends above a surface of said body of water; and (c)said borehole EM transceiver is disposed above said surface of water.21. The measurement-while-drilling system of claim 15 wherein said firstend of said signal wire is affixed to an electrode structure which iselectrically insulated from said casing string by a non-conductingsection of casing.
 22. The measurement-whiled-drilling system of claim15 comprising a second signal wire with a first end composing electrodemeans axially spaced in said borehole from said first end of said signalwire and a second end electrically connected to said surface EMtransceiver, wherein said signal wire and said second signal wirecooperate to reduce surface noise in said signal.
 23. Themeasurement-while-drilling system of claim 22 wherein said signal wireand said second signal wire are disposed within an annulus defined by anouter surface of said casing string and the wall of said borehole. 24.The measurement-while-drilling system of claim 23 wherein said surfaceEM transceiver is disposed within said annulus.
 25. Themeasurement-while-drilling system of claim 23 wherein said surface EMtransceiver is positioned above earth surface through which saidborehole penetrates.
 26. A method for transmitting an electromagneticsignal within a borehole, the method comprising: (a) providing aborehole EM transceiver; (b) providing a surface EM transceiver; and (c)reducing attenuation of said signal transmitted between said borehole EMtransceiver and said surface EM transceiver by connecting a first end ofa signal wire within said borehole to the outer surface of a tubular,and electrically connecting a second end of said signal wire to aterminal of said surface EM transceiver.
 27. The method of claim 26further comprising the additional steps of: (a) electrically connectinga first end of a ground wire to said surface EM transceiver; and (b)electrically connecting a second end of said ground wire to a groundpoint remote from said surface EM transceiver.
 28. The method of claim27 comprising the additional step of disposing said signal wire in theannulus defined by an outer surface of the tubular and the wall of saidborehole.
 29. The method of claim 27 comprising the additional step ofconnecting said first end of said signal wire near the lower end of saidtubular.
 30. The method system of claim 27 where said first end of saidsignal wire is electrically connected to the outer surface of saidtubular.
 31. The method of claim 27 where said first end of said signalwire is mechanically connected to the outer surface of said tubular. 32.The method of cam 28 where said surface EM transceiver is positionedabove ear surface through which said borehole penetrates.
 33. The methodof claim 28 wherein: (a) earth surface through which said boreholepenetrates is covered by a body of water; (b) said tubular extends abovea surface of said body of water; and (c) said borehole EM transceiver isdisposed above said earth surface and within said body of water.
 34. Themethod of claim 28 wherein: (a) earth surface through which saidborehole penetrates is covered by a body of water; (b) said tubularextends above a surface of said body of water; and (c) said borehole EMtransceiver is disposed above said surface of water.
 35. The method ofclaim 26 comprising the additional step of affixing said first end ofsaid signal wire to an electrode structure that is electricallyinsulated from said tubular by a non-conducting section of tubular. 36.The method of claim 26 comprising the additional step of reducingsurface noise in said signal by providing a second signal wire with afirst end disposed within said borehole axially spaced from said firstend of said signal wire and a second end electrically connected to saidsurface EM transceiver, wherein said signal wire and said second signalwire cooperate to reduce said surface noise.
 37. The method of claim 36comprising the additional step of disposing said signal wire and saidsecond signal wire within an annulus defined by an outer surface of saidtubular and a wall of said borehole.
 38. The method of claim 37comprising the additional step of disposing said surface EM transceiverwithin said annulus.
 39. The method of claim 37 wherein said surface EMtransceiver is positioned above earth surface through which saidborehole penetrates.
 40. A method for measuring a parameter of interestwhile drilling a borehole, said method comprising: (a) providing adownhole assembly comprising a sensor; (b) terminating said downholeassembly at lower end with a drill bit and at an upper end by a drillstring operationally attached to a drilling rig; (c) advancing saidborehole with said drill bit by rotating motion imparted to said bit;(d) providing an electromagnetic telemetry system for transmitting anelectromagnetic signal indicative of a response of said sensor, saidtelemetry system comprising (i) a surface EM transceiver for receivingsaid signal, (ii) a borehole EM transceiver for transmitting said signalwherein said borehole EM transceiver is disposed within said downholeassembly and operationally connected to said sensor, and (iii) a signalwire with a first end connected within said borehole to the outersurface of a casing string, and a second end electrically connected to aterminal of said surface EM transceiver; (e) reducing attenuation ofsaid signal transmitted between said surface EM transceiver and saidborehole EM transceiver by use of said signal wire; and (f) convertingsaid signal to said parameter of interest with a processor cooperatingwith said surface EM transceiver by means of a link.
 41. The method ofclam 40 further comprising the steps of: (a) electrically connecting afirst end of a ground wire to said surface EM transceiver; and (b)electrically connecting a second end of said ground wire to a groundpoint remote from said drilling rig.
 42. The method of claim 41comprising the additional step of disposing said signal wire within anannulus defined by an outer surface of said casing string and a wall ofsaid borehole.
 43. The method of claim 42 wherein said surface EMtransceiver is positioned above earth surface through which saidborehole penetrates.
 44. The method of claim 42 wherein: (a) earthsurface through which said borehole penetrates is covered by a body ofwater; (b) said casing string extends above a surface of said body ofwater; (c) said borehole EM transceiver is disposed above said earthsurface and within said body of water; and (d) said processor isdisposed above said surface of said water.
 45. The method of claim 42wherein: (a) earth surface through which said borehole penetrates iscovered by a body of water; (b) said casing string extends above asurface of said body of water; and (c) said borehole EM transceiver isdisposed above said surface of water.
 46. The method of claim 40comprising the additional step of affixing said first end of said signalwire to an electrode structure that is electrically insulated from saidcasing string by a non-conducting section of casing.
 47. The method ofclaim 40 comprising reducing surface noise in said signal by: (a)providing a second signal wire; (b) disposing a first end of said secondsignal wire within said borehole at a location axially spaced from saidfirst end of said signal wire; and (c) electrically connecting a secondend of said second signal wire to a second terminal of said surface EMtransceiver, wherein said signal wire and said second signal wirecooperate to reduce said surface noise in said signal.
 48. The method ofclaim 47 comprising the additional step of disposing said signal wireand said second signal wire within an annulus defined by an outersurface of said casing string and a wall of said borehole.
 49. Themethod of claim 48 comprising the additional step of disposing saidsurface EM transceiver within said annulus.
 50. The method of claim 48wherein said surface EM transceiver is positioned above earth surfacethrough which said borehole penetrates.
 51. The method of claim 40comprising the additional step of transmitting a command to saiddownhole assembly by: (a) generating a command signal indicative of saidcommand by means of input into said processor; (b) transferring saidcommand signal to said surface EM transceiver via said link; (c)transmitting said command signal with said surface EM transceiver; and(d) receiving said command signal with said borehole EM transceiver.